A lymphocyte is one of the sub-types of white blood cell in a vertebrate’s immune system. Lymphocyte includes natural killer cells, T cells and B cells. They are the main type of cell found in lymph which prompted the name lymphocyte.
Role of T. cells and B. cells:
T cells (thymus cells) and B cells (bone marrow cells) are the major cellular components of the adaptive immune response. T cells are involved in cell mediated immunity, whereas B cells are primarily responsible for humoral immunity. The function of T cells and B cells is to recognize specific "non-self" antigens, during a process known as antigene presentation. Once they have identified an invader, the cells generate specific responses that are tailored to maximally eliminate specific pathogene or pathogen-infected cells. B cells respond to pathogens by producing large quantities of antibodies which then neutralize foreign objects like bacteria and viruses. In response to pathogens some T cells, called T helper cells, produce cytokines that direct the immune response, while other T cells, called cytotoxic T cells, produce toxic granules that contain powerful enzymes which induce the death of pathogen-infected cells. Following activation, B cells and T cells leave a lasting legacy of the antigens they have encountered, in the form of memory cells. Throughout the lifetime of an animal, these memory cells will "remember" each specific pathogen encountered, and are able to mount a strong and rapid response if the same pathogen is detected again; this is known as acquired immunity.